Expression of class II antigens on ocular nonlymphoid cells was evaluated in experimental autoimmune uveoretinitis (EAU) by the immunoperoxidase technique. Class II antigens on nonlymphoid cells were not detected from normal rats. However, these antigens were detected on the retinal pigment epithelia and ciliary body epithelia a few days prior to the development of clinical and histopathological EAU. Ia antigen was noted on the retinal vascular endothelia at the onset of cellular infiltration in the retina, and appeared on the corneal keratocytes and scleral fibroblasts after the early stage of clinical EAU. The study demonstrates that during the course of EAU the ocular nonlymphoid cells can be activated to express class II antigens. This antigen expression may be important in the initiation and perpetuation of immune reactivity in the eye.